The goal of this investigation is to identify the relationship between cellular aging and specific cell surface proteins or antigens. To pursue this work we will use human diploid fibroblasts at various stages during their in vitro lifespans. Cell surface proteins will be isolated from membranes and from conditioned media and analyzed by a number of techniques including polyacrylamide gel electrophoresis and affinity chromatography. Antisera to young and old cells will be produced via the lymphocyte hybridoma-monoclonal antibody proceedure, and used to identify age-specific markers at the cell surface.